State law does not require private employers to reimburse employee expenses except as outlined in the FLSA. Per the Washington Dept. of Labor & Industries, "Reimbursement or fixed rates paid for meals and lodging while traveling, often called per-diem, is not required by state law. Additionally, reimbursements for fuel, parking fees, tolls, or other purchases made by the employee for the business are benefits given by the business at its own discretion." Unreimbursed expenses are tax deductible for employees in some cases.
State employees in Washington are eligible for reimbursement for approved and documented expenses related to official business travel.